Ministry of Science & Technology
India's SARAS radio telescope provides astronomers clues to the nature of Universe's first stars and galaxies
Scientists have determined properties of radio luminous galaxies formed just 200 million years post the Big Bang, a period known as the Cosmic Dawn thus providing an insight to the properties of the earliest radio loud galaxies that are usually powered by supermassive black holes.
Humans in their curiosity about how the early stars and galaxies formed and what they looked like have tried to capture the faint signals arising from the depths of the cosmos through a number of ground and space-based telescopes peering into the sky for a better understanding of the Universe.
Shaped Antenna measurement of the background Radio Spectrum 3  telescope -- indigenously designed and built at Raman Research Institute -- was deployed over Dandiganahalli Lake and Sharavati backwaters, located in Northern Karnataka, in early 2020.
In a first-of-its-kind work, using data from SARAS 3, researchers from the Raman Research Institute , Bengaluru, the Commonwealth Scientific and Industrial Research Organisation  in Australia, along with collaborators at the University of Cambridge and the University of Tel-Aviv, estimated the energy output, luminosity, and masses of the first generation of galaxies that are bright in radio wavelengths.
Scientists study the properties of very early galaxies by observing radiation from hydrogen atoms in and around the galaxies, emitted at a frequency of approximately 1420 MHz. The radiation is stretched by the expansion of the universe, as it travels to us across space and time, and arrives at Earth in lower frequency radio bands 50-200 MHz, also used by FM and TV transmissions. The cosmic signal is extremely faint, buried in orders of magnitude brighter radiation from our own Galaxy and man-made terrestrial interference. Therefore, detecting the signal, even using the most powerful existing radio telescopes, has remained a challenge for astronomers.
Results from the paper by Saurabh Singh from RRI and Ravi Subrahmanyan from CSIRO published in the journal Nature Astronomy on November 28, 2022, have described how even non-detection of this line from the early Universe can allow astronomers to study the properties of the very first galaxies by reaching exceptional sensitivity.
"The results from the SARAS 3 telescope are the first time that radio observations of the averaged 21-centimeter line have been able to provide an insight to the properties of the earliest radio loud galaxies that are usually powered by supermassive black holes," said Subrahmanyan, former director of RRI and currently with Space & Astronomy CSIRO, Australia, and an author of the paper. “This work takes forward the results from SARAS 2, which was the first to inform the properties of earliest stars and galaxies.”
"SARAS 3 has improved our understanding of astrophysics of Cosmic Dawn, telling us that less than 3 percent of the gaseous matter within early galaxies was converted into stars, and that the earliest galaxies that were bright in radio emission were also strong in X-rays, which heated the cosmic gas in and around the early galaxies," said Singh, one of the authors of the paper titled 'Astrophysical Constraints from the SARAS 3 non-detection of the Cosmic Dawn Sky-Averaged 21 cm Signal'.
In March this year, Singh, along with Subrahmanyan and SARAS 3 team, used the same data to reject claims of the detection of an anomalous 21-cm signal from Cosmic Dawn made by the EDGES radio telescope developed by researchers from Arizona State University  and MIT, USA. This refusal helped restore confidence in the concordant model of cosmology that was brought into question by the claimed detection.
In March this year, Singh, along with Subrahmanyan and SARAS 3 team, used the same data to reject claims of the detection of 21-cm signal from Cosmic Dawn made by the EDGES radio telescope developed by researchers from Arizona State University  and MIT, USA.
"We have now got constraints on the masses of the early galaxies, along with limits on their energy outputs across radio, X-ray, and ultraviolet wavelengths," Singh noted. Further, using a phenomenological model, SARAS 3 has been able to put an upper limit to excess radiation at radio wavelengths, lowering existing limits set by the ARCADE and Long Wavelength Array  experiments in the US.
"The analysis has shown that the 21-cm hydrogen signal can inform about the population of first stars and galaxies ", shared another author, Dr. Anastasia Fialkov from the Institute of Astronomy, University of Cambridge. "Our analysis places limits on some of the key properties of the first sources of light, including the masses of the earliest galaxies and the efficiency with which these galaxies can form stars," said Fialkov.
Since its last deployment in March 2020, SARAS 3 has undergone a series of upgrades. These improvements are expected to yield even higher sensitivity towards detecting the 21-cm signal. Currently, the SARAS team is assessing several sites in India for its next deployment. "These sites are fairly secluded and pose several logistical challenges for deployment. However, they seem promising from science's viewpoint and, with new upgrades, seem ideal for our experiment", adds Yash Agrawal, a member of the SARAS team.
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